| The India-Eurasia collision about 50Ma ago and the continued northward indentation of India has resulted in a conspicuous uplift of the Tibetan plateau as a whole, where the continental lithosphere is experiencing most intensive deformation. It is one of the most mysterious geological events since late Tertiary time in Asia and the world. Thus this region has become an idealized field laboratory for continental dynamics. The Sichuan-Yunnan region in the eastern margin of the Tibetan plateau is a transitional zone from highland to basin in tectonics because of the India-Eurasia collision and the eastward indentation of blocks in the Tibet plateau, which is characterized by steep gravity gradients from south to north and aeromagnetic anomaly belt, large variations of crustal thickness, strong crutsal movements, active faults and frequent earthquakes as well as metallic mineralization zones. GPS data reveal that the crustal motion turns to east and southeast in this region, which may imply an eastward flow of the lower crust material there.It is, however, not clear what is the deep background for the dynamic processes mentioned above. Hence to probe the structure beneath this region by geophysical, including magnetotelluric (MT) sounding, is an urgent task. This thesis focuses on analysis of the MT data collected on the Mianning-Yibing profile across the eastern margin of the Tibetan plateau, which is the longest peofile in the eastern margin of Tibet plateau, and the first study to detect the deep structure in this area.The purpose in this project is to infer deep structure beneath the study area and study whether there is a lower-crust flow layer and its implications below this area. In this thesis, the data analysis from qualitative to quantitative with the advance MT data processing technique and 2D inversion are performed to yield fine electric structure.In conjunction with the Shimian-Leshan profile in the north and Meigu-Suijiang profile in the sourth, this thesis suggests the MT sounding evidence for the channel flow layer below the study area. Consideration of the electrical structures of other profiles in this region, a detailed discussion is made on the structural characters and contact relationships among the blocks. In combination with other data (geology, seismology, heat flow, and GPS), a kinematic model is suggested for this region and a further analysis is made on seismicity in this region.1 MT data analysis and its applicationThe eastern margin of Tibet plateau is the area with complicated structures and strongly deformed strata and development faults especially where the Mianning-Yibin profile crosses. It’s important to analysis, distinguish and decrease the affect of the distoration by the complicated structures when the 2D inversion is used today and 3D inversion is not applied.1.1 Impedance tensor decomposition and its application The impedance tensor was adopted in the sixth decade of last century, and took place of the impedance scalar quantity (Hermance,1973) in order to use the 2D MT data observation, processing and interpration. The local distortion (Berdichevsky,1998) caused by 3D near-surface abnormal bodies has been a difficult issue in MT data interpretation since a long time.In the eighth decade of last decade, methods of impedance decomposition were put forward as an effective tool to tackle this problem (Swift(1967);Bahr(1988); Groom&Baily(1989); Zhao(1996); Wang (2001); Jin (2003))).This work uses the impedance decomposition methods suggested by Bahr and Groom&Baily to make a comparative analysis of measurement data along the profile and contrast to the traditional method by Swift. The result shows that the distoration caused by the local anomaly body is little for majority of sites, and the true electrical structure can be acquired through the Swift analysis method. The azimuth of the principal axis on deep electrical structure and the correlative distortion parameters can be gotten by the GB impedance decomposition method to the some distorted sites.The reasonable apparent resistivity and impedance phase can be acquired with the azimuth of the principal axis and other distortion parameters.The 3D local distoration can be avoided to get the true electrical structure and the reason about the distoration is analyzed combined with the observation data.1.2 Influence of faults on MT dataThe influence of faults with variational width, depth and tendence on the MT data is calculated in this thesis in order to anyalyze the influence of different faults with the 2D forward modeling. Forward modeling shows that lithology variation between both sides of a fault can produce major influence on apparent resistivity curves and there’s a bigger leap transform along the appearent resistivity curves in the TM mode than TE mode, while the width of the fault does not seiriously. But such influence would enhance with increasing depth of the fault and cause the appearent resistivity change with the varying tendence of the fault. It would give a good help and instruction to the MT data observation in the area with development structures.1.3 Influence of topography on MT dataThe relatively simple model with a low resistivity horst below the valley and the peak model is calculated with forward methods in order to anyalyze the influence of different topography.The forward modeling on relatively simple models with valley and peak topography indicates that for 2D inversion of apparent resistivity and impedance phase, the low-resistivity body below the relief shifts upward when the inversion does not include topography, which has some effect on the underlying strata, easily to produce redundent structure and big RMS in final data fitting. When the topography is considered in NLCG inversion, the low-resistivity body in the lower part of the model with a valley and peak can exhibit good response, consistent with its real position and its lower boundary can also be well revealed. It demonstrates that the NLCG inversion with consideration of topography can overcome the influence of relief effectively and acquire relatively true electric structure in the subsurface.1.4 Information from the MT data and reasonable metoods used in 2D inversionThe 2D skewness, azimuth of the principal axis, magnetic induction vectors, apparent resistivity and impedance phase are acrquired from the MT data. It’s very important to define and get the true electrical structure when these informations are analysed and adopted in the data inversion calculation especially in mountainous areas where the Mianning-Yibin profile cross. With this analysis, some characteristics of subsurface electric structure can be determined, such as segmentation, layering, complexity, electric boundaries, dimension and orientations of principal axis, which not only provide evidence for the reseasonable parameters and data selected in 2D inversion and interpretation but also can be combined in the 2D inversion to proof the true electrical structure and reseanable interprations.This work analyses the inversions on different models including TE mode,TM model and the joint inversion of the TE/TM mode, and select the joint inversion of the TE/TM mode as the final result.Further more, the last inversion parameter selected and mesh grid subdivision are determined through lots of tests on the inversion model mesh grid subdivision density,data error selected and the model smooth regularization factor.The accepted model in this thesis is acquired with the joint inversion of TE &TM mode data by the NLCG method with topography.2 Two-D inversion of Mianning-Yibing profile and analysis of electric structureA stepping maner is adopted in order to get a stable convergent result and avoid falling into a local extreme value or reduce unexpected structure during Mianning-Yibin MT profile inversions. For example, a smooth model of low resolution is firstly established using a big regularization factor, which outlines the realistic model. Then this resulted model is used as the initial one and the value of the regularization factor is lowered for the next inversion. This process is repeated till the data fitting meets the requirement. The inversion mesh grid is changed in this process too.It indicates from the electrical structure along the Mianning-Yibin profile that the section can be devided into three parts.The first is Kangdian geo-axis, the next Daliang Shan block and the third Sichuan basin from west to east.Their boundaries are Daliang Shan fault and Ebian fault respectively.The west boundary of Kangdian geo-axis locates inside the Sichuan-yunnan block, and the eastern is Daliang Shan fault. The upper crust of the Kangdian geo-axis is featured by high resistivity, at thickness thinner from west to east. There is a low resistivity layer below the upper crustal high resistivity layer. The upper crust of the Daliang Shan block is featured by high resistivity too. But the resistivity is less than Daliang Shan block’s, and the thickness is thinner than Daliang Shan block’s.There is a low resistivity layer connectting with the Kangdian geo-axis’ below the upper crust. The crust of Sichuan basin as a whole is highly resistant and no low-resistivity layer below the upper crust. An obvious character along the profile is that the low-resistivity layer of the middle-lower crust in the Kangdian geo-axis and Daliang Shan block looks as an upward arch architecture whose top is located in the middle of Daliang Shan block.Furthmore, the Anning He fault, Ganluo fault, Xihe-Meigu fault and Daliang Shan faults exhibit nearly vertical and broad zones of low-resistivity anomalies, whose bottom interfaces disappear in the top of the high-conductive layer of the crust. The high-conductivity layer in the middle-lower crust is presumably attributed to the partial melting or salt-bearing fluids involved.3 Electric structure, dynamic model and Seismicity in the eastern margin of the Tibetan plateauFrom the Mianning-Yibing MT profile, in conjunction with other four profiles in this region (Kangding, Shimian-Leshan, Meigu-Suijiang, and Qiaojia profiles), a fairly complete image of electric structure in the eastern margin of the Tibetan plateau has been acquired. It indicates that along the Kangdian geo-axis, from north to south, the upper crust exhibits a high-resistivity layer of over thousandΩm and the thickness of about 30-40km which becomes thinner toward east. In geology this layer represents the old metamorphic rocks and volcanic rocks in upper crust, as well as the crystalline basement consisting of highly metamorphic old strata. Below this high-resistivity layer is a 20～25 km thick layer of low resistivity, which crosses the Anning He fault and Daliang Shan fault and links the high-conductivity layer in the Daliang Shan block. Downward farther, resistivity increases to some degree.The electric resistivity of the upper crust in the Daliang Shan block is lower than that in the Kangding bock, probably associated with the thick carbonate and evaporite of Paleozoic and Mesozoic times there. Beneath them is a upward convex HCL, whose top is at depth as shallow as 5km in the middle of the Daliang Shan block. Eastward farther, this HCL tends to be deeper with the largest buried depth about 30km, and seems to underthrust toward the Sichuan basin. The thickness of HCL is general at 25km, with a consistent way especially on the Shimian-Leshan and Mianning-Yibing profiles, while not so obvious on the Meigu-Suijiang profile. Beneath this HCL the resistivity increases again by tens to hundredsΩm in general.Along the Mianning-Yibing and Meigu-Suijiang profiles, the electric structure of Sichuan Basin can be divided into three layers. The first layer is further subdivided into upper and lower sublayers.The upper one exhibits an alternating high-and low-resistivity pattern in lateral direction.The lower one is of relatively low resistivity. The bottom of the first layer is 8 km deep in the west, and increases to 15km in the east. The second layer is thick and highly resistive.The depth and thickness tends to increase toward east, corresponding to the middle and lower crust without inner high-conductivity layer. Below this high-resistivity layer, resistivity values rise up again down to the uppermost mantle. The electric structure of this area suggests that there exists thick and hard crust beneath the thick sediments in Sichuan basin.The nearly vertical Arming He fault coincides with a vertical zone of low resistivity, which is cut into upper and lower parts by a high-conductivity layer in the middle-lower crust. The upper part is 5km wide. The resistivity is several hundredΩm and smaller than that on the east and west sides.The bottom disappears in the top of a low-resistivity layer in the middle crust. Below this low-resistivity is a nearly vertical boundary of electricity, where the resistivity on the east side is higher than the west side. Along the Shimian-Leshan profile, the Anning He, Daliang Shan and Ganluo faults merge at one place whose depth is shallower then that along the Mianning-Yibing profile above the high-conductivity layer. The Daliang Shan fault also exhibits a nearly vertical zone of low resistivity which is several kilometers wide and the bottom linking with the low-resistivity layer in the crust. The Ebian fault corresponds to the boundary between two blocks with big differences of resistivity on either side, which tends to dip eastward.With reference to the hypothesis of the channel flow of lower crust in the eastern margin of the Tibetan plateau, as well as integrated data of the electric structure and GPS measurements, this thesis suggests a model of high-conductivity layer flow in crust in the study region which is described as follows. Due to the effect of the India-Eurasia collision in the south, the Tibetan plateau moves toward north. Because of the impeding of the Tarim block in the north, movements of massifs in the plateau turn to east with respect to South China. In the eastern margin of the plateau, the motion direction of the high-conductivity layer in crust changes from eastward to southeastward because of the obstruction of the Sichuan basin. As well as the HCL plunge down into the beneath of the Sichuan basin.Data analysis suggests that the areas with lateral variations of conductivity of low-resistivity layers in crust are usually active in seismicity in this region. For example, Major events as well as dense small quakes are distributed along Anning He fault. Around the Daliang Shan fault, small earthquakes exhibit a concentrating pattern. The Ebain fault is also an electric boundary.While within the Daliang Shan block, a few medium-sized quakes are documented in history and small events are less with respect to the boundary zones on its both sides.In the Songpan-Ganzi block, the high-conductivity layer moves in SSE-SE direction, crossing the 300km-long Longmen Shan fault zone in a normal direction and is hampered by the Sichuan basin. Thus a stable "T" shaped structure is formed at the central portion of the Longmen Shan which makes motion and deformation difficult, and stress easy to build up but not easy to release. Consequently, before the Wenchuan M8 event of 2008, the Longmen Shan fault zone was characterized by a low slip rate and weak seismicity. Once the long-term accumulation of stress exceeded the rupture strength of crustal rock beneath the Longmen Shan, sudden fractures and slips occurred to produce the Wenchuan quake in 2008. |
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